Method and device for the preparation of foundry sand

ABSTRACT

The invention relates to a method for the preparation of foundry sand by means of a mixing process which is carried out in a mixer ( 1 ), said preparation occurring at least partially in a vacuum. In order to provide a method and device for the preparation of low-cost foundry sand which can be used in an efficient manner, whereby said foundry sand has a uniform temperature and homogeneous quality in addition to being able to be charged more quickly and therefore economically in comparison with other mixers, the foundry sand is added, at least intermittently, in the form of a volume flow of at least  100  l/s through an opening in the mixer which has a cross-sectional area of at least 0.25 m 2 , preferably at least 0.4 m 2 , more preferably at least 0.5 m 2 .

[0001] The present invention relates to a method and a device for thepreparation of mould sand using a mixing process in a mixer, wherein thepreparation takes place at least partially under vacuum.

[0002] The preparation of sand for the manufacture of moulds is intendedto produce the correct mixing ratio of grain sizes and of proportions ofquartz sand, binding agent, coal dust, optionally other additives aswell as recycled and new sand, to homogenise the mixture and, in thiscontext, to coat the grains with the binding agent to a large extent, toadjust the correct moisture content, to adjust the correct temperatureof the mould sand and finally to convey the ready-prepared sand to theplaces of use.

[0003] Recycled sand generally has an increased temperature, forexample, of between 100° C. and 140° C. Because sand temperatures aboveapproximately 50° C. can present considerable problems to the mouldingequipment, and at excessively high temperatures, uncontrollableevaporation losses over the distance between the mixer and the mouldingequipment can lead to fluctuations in moisture in the ready sand, thesand must be cooled in this case. Conveyor belt coolers, through whichthe sand passes continuously as a result of vibrating or agitatingmovements of a screen mesh, are mostly used for this purpose.

[0004] An alternative cooling method has been proposed in DE 295 24 03C2. This cooling method provides the concurrent preparation and coolingof clay-bound foundry moulding sands in a vacuum mixer. In this context,the individual components are initially placed into the mixer. After ashort prehomogenizing process, the temperature and moisture of themixture are measured and the necessary quantity of water is added.Finally, during the preparation process, the pressure in the mixture isgradually reduced. As soon as the pressure corresponds to the vapourpressure curve of water, the water in the sand begins to boil andremoves the necessary evaporation heat from the sand. As a result, anextremely effective cooling method is achieved in an economical manner.According to the disclosure of DE 199 45 569, alongside the coolingeffect on the mould sand, the described cooling of mould sand undervacuum also leads to an increase in quality of the mould sand prepared.Accordingly, DE 199 45 569 suggests preparation under vacuum even forrecycled sand which has already been cooled.

[0005] It has been shown that the best mould sand quality can beachieved with the assistance of vacuum preparation. However, the knownstages of the method and the devices and/or peripheral equipment usedand their method of operation are only suitable to a limited extent foruse in a fully automated foundry moulding plant. According toexperience, error-free and above all economically optimized operation isnot possible with the known method.

[0006] This is because, among other factors, the filling and emptying ofthe mixer is very time-consuming. For the ventilation of the mixer, amixer cover is provided, in all known versions, which, in the closedcondition, must be vacuum-tight to allow vacuum operation, and which isopened for the purpose of charging the mixer. In this context, the coveris generally connected to the mixer in a rotatable manner by means of arotating axis. The cover can be designed in such a manner that it isrotated outwards or inwards in order to open the container. If it isrotated inwards, the closing mechanism must press the cover outwardsagainst the sealing surface of the mixer with considerable force duringvacuum operation. In order to manufacture the closing mechanism in aneconomical manner, the mixer cover must therefore be very small, becauseonly a small force need then be applied by the closing mechanism.

[0007] If the cover is opened outwards, the closing mechanism can bedesigned to be weaker and can therefore be manufactured moreeconomically, because the necessary pressing force can be generated bythe pressure difference between the mixing container and thesurroundings alone. However, with this version, the design must takeinto consideration that sufficient rotational clearance remains abovethe cover so that the cover can be opened without it coming into contactwith any objects. Accordingly, dosage funnels or dosage devices must beattached at an appropriate distance above the mixer opening. Thisdistance necessarily increases in proportion with the size of cover.However, when filling the mixer, it is important to ensure that thesealing surface of the charging opening remains as a free fromcontamination as possible, in order to guarantee a vacuum-tightclosability. However, the probability of contamination of the sealingsurface increases considerably as the distance between the dosage funneland filling opening and/or the dropping height of the substance to becharged increases. For this reason, it is currently assumed, that ageneric mixer cannot be manufactured economically with a large chargingopening. Accordingly, the known mixers all possess only a relativelysmall opening in the pressure casing of the mixer, and, in the case ofthe equipment known so far, the mixture is added only in a very fineflow. This results in a very long charging time and therefore also avery low plant performance. If the mixer is loaded too quickly, anexcess air pressure rapidly builds up as a result of compression of theair in the mixer. This excess pressure generally leads to the emissionfrom the mixer of dust-like components of the mixture, which can, forexample, also be deposited on very sensitive machine components, such ascogwheels and gaskets. This means that the plant must be cleaned morefrequently which, once again, is associated with higher costs andundesired interruptions of operation. For this reason, it has generallybeen considered that, on the one hand, the charging rate cannot befurther increased, because larger input openings cannot be realized inan economical manner. And, on the other hand, it has been consideredthat a larger charging rate will lead to the disadvantages described andmust therefore be avoided.

[0008] The object of the present invention is therefore to provide amethod and a device for preparation of mould sand, which can be usedeconomically without errors, and provides mould sands of uniformtemperature and uniformly high quality in an economical manner andwhich, in comparison with the known mixers, has an increased chargingrate.

[0009] With regard to the method, this object is achieved by thematerial to be charged being added at least at times in a volume flow ofat least 100-800 l/s through an opening with a diameter of at least 150mm, preferably at least 300 mm, and particularly preferably at least 500mm.

[0010] In this context, the pressure difference between the ambientpressure and the pressure in the mixing chamber of the mixer ispreferably used either as the exclusive or predominant drive for atleast one feeder process for water or for a mixture component or foraccelerating the feeder process. According to the invention, therefore,the dosage devices and charging devices are coupled with the mixing unitin terms of the design and method. As a result, the vacuum prevailing inthe mixer can be used, for example, to accelerate the chargingprocesses, and even during the charging phase, to improve thedistribution of the additives and the fluids to be added. In particularin combination with a large charging opening, the consistent use ofpressure difference can achieve a significant reduction in chargingtime. Moreover, this method causes no additional costs, because theevacuation device necessary for the preparation method is alreadypresent.

[0011] Particularly preferred is a method in which at least a portion ofthe quality-determining mixture components is fed into the mixer duringthe charging and/or mixing process. The quality-determining mixturecomponents are the additives already mentioned, such as, for example,bentonite, coal dust, etc., which are added to the recycled sand, inorder to adjust the quality of the prepared mould sand. The vacuumpressure in the mixer, which is used to suck in the components to befilled, also effectively prevents the emission of dust-like mixturecomponents from the mixer and their being deposited, for example, onsensitive machine parts.

[0012] A particularly expedient version of the method according to theinvention additionally provides that the individual mixture componentsare introduced into the mixer one after another according to apredetermined sequence. However, for special cases of application, itmay be advantageous if the water is only introduced into the mixer afterthe other mixture components have been introduced into the mixeressentially concurrently. As a result, after the other mixturecomponents have been introduced, it is possible to determine theresidual moisture and temperature of the recycled sand and from this, tocalculate the appropriate quantity of water to be added.

[0013] In particular, in order to be able to mix the water to be addedas well as possible with the mixture, a preferred version of the methodprovides that at least one portion of the water to be added isintroduced directly into the mixture with the assistance of a preferablyrotating feeder device. In this context, by rotating is meant rotatingrelative to the mixer, so that it is irrelevant whether the feederdevice rotates or the feeder device remains stationary and the mixerrotates around the stationary feeder device, or whether both the mixerand the feeder device rotate. This direct method of feeding, that is,below the mixture level, can enable a very good mixing of the water withthe mixture.

[0014] A particularly effective version of the method provides that atleast one portion of the water is introduced into the mixture via afeeder device, which is coupled with a mixing tool or even integratedinto a mixing tool. This is particularly advantageous if the mixeralready provides a mixing tool. Moreover, as a result of this stage ofthe method, the water can be mixed directly with the filling substance.

[0015] It is also particularly expedient if the quality-determiningmixture components, such as e.g. bentonite and coal dust, are introducedinto the mixer below the filling level of the mixture. This measure alsoguarantees a very good mixing of the quality-determining mixturecomponents with the mixture in the mixer.

[0016] The quality-determining mixture components are preferablyintroduced centrally and directly inside the vertically and tangentiallyflowing mixture bed. This further enhances miscibility.

[0017] For some cases of application, it may also be advantageous if atleast one portion of the quality-determining mixture components is firstmixed with air, and this air/solid mixture is then introduced into themixer, i.e. preferably below the filling level. After preparation of themould sand, the mixer must necessarily be ventilated again, that is, thepressure difference between the mixing container and the ambientpressure must be equalized. This can be achieved, for example, simply byopening the container cover. However, a method, in which the mixingchamber is ventilated via a feeder which terminates in the mixingchamber below the filling level of the mixture, is particularlypreferred. As a result, the mould sand is compressed to a lesser extent.By contrast, if the equalization air is fed in above the sand level, akind of pressure cushion will be formed on the surface of the sand as aresult of the pressure difference prevailing above and below the fillingsubstance, which leads to definite, temporary compression at least ofthe uppermost layer of sand.

[0018] Of course, it is also possible to use the feeder, which isprovided for feeding the quality-determining mixture components, forventilation and/or pressure equalization. With regard to the device, theobject mentioned initially is achieved by a device for the preparationof mould sand with a mixer, which has a vacuum chamber or is arranged ina vacuum chamber, which can be closed in an essentially vacuum-tightmanner, with devices for feeding the components to be mixed, at leastone mixing tool as well as a device for removing the ready mixture,wherein a closable feeder connection for the components to be mixedexists or can be manufactured between the mixing container and theoutside, wherein the feeder opening has a cross-sectional area of atleast 0.25 m², preferably at least 0.4 m² and particularly preferably atleast 0.5 m².

[0019] In principle, the feeder opening can have any cross-sectionalform, round or rectangular forms being preferred, however.

[0020] Feeding preferably takes place through the feeder opening eitherexclusively through the pressure difference between the ambient pressureand the pressure in the mixing chamber of the mixer, or feeding is atleast accelerated by this pressure difference.

[0021] Through this at least one closable feeder connection, thepressure difference between the ambient pressure and the pressure in themixing chamber of the mixer can be exploited as a driving force. If thefeeder connection is opened, as a result of the vacuum pressure existingin the mixing chamber, material to be supplied is drawn into the mixingcontainer from the outside. In general, an additional pump is notnecessary for this purpose. The feeder therefore requires no additionalenergy and is, moreover, essentially maintenance-free.

[0022] Particularly preferred is a version in which an essentiallyvacuum-tight, closable filling opening of the mixer can be connected viaan essentially vacuum-tight intermediate space to the outlet opening ofat least one dosage device which is preferably designed as a dosageweighing device. Recycled sand, for example, can be introduced into themixer through this opening. For this purpose, the mixer must first beplaced under a vacuum. Following this, the filling opening of the mixeris opened, so that the mixing chamber is connected to the essentiallyvacuum-tight intermediate space. After this, the outlet opening of atleast one feeder device is opened so that the charging materials fromthe feeder device are first conveyed into the intermediate space andthen into the mixing chamber. This charging takes place very quicklybecause the pressure in the mixing chamber of the mixer and in theintermediate space is significantly lower than the pressure in thefeeder device. Particularly preferably, the filling opening of the mixerand/or the outlet opening of the feeder device have a cover with lateralcheeks, which, with the assistance of the lateral cheeks, forms a kindof conveyor chute in the opened state. With the assistance of thisconveyor chute, the supplied filling materials can be conveyed from theoutlet opening of the feeder device at a high flow rate directly intothe inlet opening of the mixer. Particularly preferably, the inletopening of the mixer and also the outlet opening of the feeder devicehave a cover with lateral cheeks, each of which therefore form aconveyor chute in the opened state.

[0023] For particular cases of application, it could be advantageous ifa further movable chute component is provided, which can be movedindependently of one of the covers. For the purpose of feeding in thiscase, the cover of the inlet opening of the mixer can first be opened,preferably by means of a control device, then the movable chutecomponent can be brought into its functional position and finally thecover of the feeder device can be opened. The three chutes are thenpreferably arranged in such a manner that they form a passage for thesubstance to be filled and guarantee a rapid and targeted charging ofthe mixer with the substance to be filled. The chutes are preferablyarranged in such a manner that they project into the openings andthereby prevent the substances to be filled from coming into contactwith the edges of the opening. Contact of this kind could, under somecircumstances, impair the sealing function of the cover of the inletopening of the mixer.

[0024] A particularly preferred version of the device according to theinvention provides that the mixing chamber of the mixer is arranged in apressurized container and that, inside the pressurized container butoutside the mixing chamber, a closable air feeder is provided. Thepressurized container is advantageously connected via appropriate sealsto the mixing chamber arranged in the pressurized container. These sealsnecessarily allow the passage of air, but are intended to hold thecomponents of the mixture in the mixing chamber as firmly as possible.It is not desirable for mixture to escape from the mixing chamber intothe pressurized container because contamination of the seals and movabledrive components and bearings can occur there. If the mixing chamber isthen rapidly loaded with the substance to be filled without vacuum, thepressure in the mixing chamber will rise very rapidly. The sealsgenerally used between the mixing chamber and the pressurized containerare, however, not capable of sustaining their sealing function undersuch an abrupt rise in pressure. It can therefore occur that materialfrom the mixing chamber, which then has a higher pressure than thepressurized chamber, enters the pressurized chamber. As a result of theclosable air feeder arranged according to the invention inside thepressurized container but outside the mixing chamber, the pressure inthe pressurized container outside the mixing chamber can be increased bythe air feeder during the charging process, so that the pressure in thepressurized container is higher than pressure in the mixing chamber. Inthis way, any transfer of material from the mixing chamber into thepressurized chamber is prevented.

[0025] Particularly preferred is a version in which a control unit isprovided which opens the air feeder when the mixture components aresupplied and closes the air feeder when the vacuum container is closedin a vacuum-tight manner. This control unit is preferably automated, sothat, depending on the stage of the method, it is possible to evacuatethe pressurized container and also to allow a build-up of pressure inthe pressurized container, in order to counteract any abrupt rise inpressure in the mixing chamber as a result of the feeding of mixturecomponents. It is self-evident that the air feeder described can be usedoutside the mixing chamber but inside the vacuum chamber even with knownmixers. Even if the known mixers do not use the pressure differencebetween the pressurized chamber and the surroundings as a driving force,and consequently, the abrupt rise in pressure resulting from thecharging process is significantly smaller, a transfer of material fromthe mixing chamber into the pressurized chamber will be prevented evenwith known mixing containers.

[0026] A further particularly preferred version of the present inventionprovides that a feeder device for water is arranged in such a mannerthat the water is supplied through or along a preferably eccentricallyarranged mixing tool with mixing paddles and fed into the mixtureessentially in the region of the ends of the mixing paddles. Also inthis case, according to the invention, the pressure difference betweenthe mixing chamber and the outside environment is exploited. If water isto be fed into the mixture, a valve merely has to be opened. As a resultof the vacuum pressure prevailing in the mixing chamber, the water issucked through the feeder device directly into the mixture. Thearrangement of the feeder device along a mixing tool has the advantagethat the water can be fed directly into the mixture at differentpositions. In this context, the fluid outlet openings in the waterfeeder device are preferably arranged at different depths below thelevel of the mixture. Adequate mixing can therefore be achievedextremely quickly.

[0027] Particularly preferably, the feeder device for water has a dosageweighing device, the dosage weighing device and mixer being connected bya preferably at least partially resilient pipe, which can be closed by avalve, the valve being preferably arranged on the cover of the mixer.

[0028] The so-called quality-determining mixture components arepreferably fed with the assistance of a feeder lance, if possible belowthe level of the mixture. In this context, the outlet opening of thefeeder lance is, if possible, oriented tangentially to the direction offlow of the mixture and preferably points in the direction of flow. Thisguarantees that, as a result of the flow of mixture emphasized by therotation of the mixture, the quality-determining mixture components,which are sucked into the mixing chamber as a result of the vacuumpressure prevailing in the mixing chamber, are drawn along with themixture in the direction of flow and are rapidly and effectively mixedwith the latter.

[0029] Further advantages, features and possible applications of thepresent invention are explained with reference to the followingdescription of preferred versions and the associated figures.

[0030] There are shown in:

[0031]FIGS. 1a) and 1 b) a lateral view of the arrangement of a feederdevice and a feeder opening of the mixer in an opened and in a closedposition,

[0032]FIG. 2 a lateral view of a vacuum mixer with a partial sectionalview,

[0033]FIG. 3 a detailed view of FIG. 2,

[0034]FIGS. 4a) and 4 b) a schematic representation of the connectionbetween a fluid dosage weighing device and mixing container,

[0035]FIGS. 5a) and 5 b) a lateral view and a top view of the feederelements for fluids in the mixing chamber,

[0036]FIG. 6 a schematic representation of the feeder for thequality-determining components and

[0037]FIG. 7 a lateral view of the arrangement of an alternative feederdevice and a feeder opening of the mixer.

[0038]FIGS. 1a) and 1 b) show the outlet region of a solids weighingdevice 10 and the inlet region of the mixer 1. The solids weighingdevice 10 is used for determining the quantity of recycled sand to besupplied or optionally also of other mixture components. In FIG. 1a),both the mixer 1 and the solids weighing device 10 are closed, whileFIG. 1b) shows the conveyance position between the mixer 1 and thesolids weighing device 10.

[0039] An inlet connecting piece 2 is arranged at the upper side of themixer 1. This inlet connecting piece 2 is sealed in a vacuum-tightmanner through the container cover 3 with the assistance of the leverarm 5, which is driven, for example, by a hydraulic cylinder. It can beseen that the container cover 3 provides one lateral cheek 4 at both ofits lateral outer edges.

[0040] The solids weighing device 10 also has an outlet flap 11, whichprovides lateral cheeks 11′ at both of its lateral outer edges. Thisflap can be opened or closed via the lever 12.

[0041] Moreover, this version has a conveyor chute 13. The conveyorchute 13 also has lateral cheeks 13′ at both of its lateral outer edges.The conveyor chute 13 can be moved with the assistance of the parallelguide 14 and the lifting drive 15 into the intermediate space betweenthe solids weighing device 10 and the mixer 1. As a result of thelateral cheeks, the outlet flap 11, filling flap 3 and conveyor chute 13provide an essentially U-shaped cross-section, the lateral cheeksforming the two arms of the U-shape.

[0042] The conveyor chute 13 is arranged in such a manner that, in theextended position, when the outlet cover 11 of the solids weighingdevice 10 is open, together with the outlet flap 11 and the lateralcheeks 11′, 14′, it forms a channel with an essentially rectangularcross-section.

[0043] This channel is further extended by the open filling cover 8 withits lateral cheeks 4, so that, in the conveying position, the situationshown in FIG. 1b) is produced. In this position, the material is guidedfrom the solids weighing device 10 directly into the mixer 1. Theconveyor chutes in this arrangement form a kind of channel, so that theedges of the opening are covered and cannot come into contact with thematerial to be filled.

[0044] The entire range of movement of the flaps 3, 11 and of theconveyor chute 13 is surrounded by a housing 6 and/or 6′. In the versionshown, the housing is designed in two parts; and the two housing parts6, 6′ are connected to one another via a flexible preferably sealingconnection 7.

[0045] The charging process is as follows. First, the two covers 3, 11of the mixer 1 and the solids weighing device 10 are closed. If themixer is to be charged with materials which are located in the solidsweighing device 10, the cover 3 of the mixer 1 is first opened. Next,the conveyor chute 13 is moved into the region between the solidsweighing device 10 and the mixer 1. This has not hitherto been possiblebecause, in the extended state, the conveyor chute 13 is located withinthe rotational range of the filling cover 3 of the mixer 1. If theoutlet flap 11 of the solids weighing device 10 is then opened, thematerials from the solids weighing device will be filled directly andrapidly into the mixing chamber of the mixer 1 via the channel formed bythe outlet flap 11, inlet cover 3 and conveyor chute 13. In this manner,the mould sand from the solids weighing device 10 is conveyed into themixer 1 without significant material loss and without substantialemissions of dust, through a large cross-section and in a short time.

[0046] As shown in the diagrams, additionally arranged in the housing 6,6′ are air nozzles 8, 9 which direct an air-flow onto the seal of theinlet cover 3 and the drive mechanism of the inlet cover 3, so that,after each filling process, those places are fanned on which a depositof sand may have a negative effect, in order to guarantee a secure andsealed closure of the inlet cover 3.

[0047] The inlet cover 3 of the mixer 1 according to the invention doesnot have any particularly complicated design for the sealing elements.Rather, it is simply pressed by the vacuum pressure prevailing in themixer 1 against the opening of the mixer 1, so that the opening or thecover 3 merely needs to be surrounded by a sealing ring. However, thisversion of the inlet cover 3 necessarily requires a certain distancebetween the solids weighing device 10 and the mixer 1, because the cover3 requires sufficient clearance for rotation. As described, thisdistance is bridged by the material guiding channel, which is formedfrom the flaps 3, 11 and the conveyor chute 15 and lateral cheeks 4,11′, 13′. According to the invention, the charging time for a mixer ofthis kind is reduced from approximately 30-40 seconds, which is entirelystandard for mixers available on the market, to less than 10 seconds.

[0048] The mixing chamber 16 of a vacuum mixer 1 is normally arranged ina vacuum chamber 17. The basic structure is shown in FIG. 2 with furtherdetail shown in FIG. 3. The vacuum chamber 17 is sealed via flexibleseals 18 from the mixing chamber 16. In this context, the seal 18 isused only to prevent the entry of mixture material from the mixingchamber 16 into the vacuum chamber 17. The drive unit for the mixer isgenerally arranged in the vacuum chamber 17 but outside the mixingchamber 16. For this reason, the reliable function of the seal 18 isextremely important, as otherwise the intermediate space 17 would haveto be cleaned frequently, as otherwise the drive unit could be destroyedas a result of solid materials of the mixture. The charging phase, inparticular, is a very critical moment for the seal 18. Even with theconventional mixers, an abrupt rise in pressure occurs as a result ofthe filling process, so that repeated functional failures of the seal 18occur. This problem is further intensified by the charging process asdescribed with reference to FIGS. 1a) and 1 b). According to theinvention, the mixing container is under vacuum at the beginning of thecharging process, so that the abrupt rise in pressure in the mixingcontainer is even more pronounced during charging. In order to preventthe entry of dust into the intermediate space 17, a sliding ring sealmay, for example, be used. However, since this causes very high costs, aversion according to the invention provides a closable air feeder 19.This air feeder, which in FIGS. 2 and 3 is designed as a pressure fan,is capable of increasing the pressure in the intermediate space 17 atthe beginning of the charging process. In this context, the rise inpressure in the intermediate chamber 17 should approximately correspondto or even exceed the abrupt rise in pressure in the mixing chamber 16.

[0049]FIG. 3 shows structural details of the seal 18. Approximately atthe start of the charging process, the valve 21 is opened, so that thepressure fan 19 introduces air into the intermediate space 17 betweenthe pressurized chamber wall 17′ and the mixing chamber wall 16′. Theair introduced flows in the direction of the arrow through the gap seals18, 22 into the mixing chamber 16. As a result of this measure, theemission of dust or material from the mixing chamber 16 into theintermediate space 17 is effectively prevented.

[0050] It is evident that the air feeder does not necessarily require apressure fan 19 or a similar device; for some cases of application, itmay be adequate if only a closable opening is provided as the airfeeder, which is simply opened at the beginning of the charging process,so that the pressure in the vacuum chamber and/or the intermediate space17 and the mixing chamber 16 rises in an approximately synchronizedmanner. At the start of the preparation of the mould sand under vacuum,the air feeder must be switched off again or closed.

[0051]FIGS. 4a) and 4 b) show the charging of the mixer with thenecessary quantity of mixing water. Between 0.5 and 4% mixing water arenormally added to the mixture. The exact quantity of water to be addedis determined by measuring the residual moisture of the recycled sandbefore placing in the mixer or even in the mixer. The residual moistureof the recycled sand and therefore also the quantity of mixing water tobe added depends on the preliminary thermal loading of the recycledsand.

[0052] Moreover, it must be borne in mind that the vacuum-coolingprocess also consumes a certain quantity of water because, as describedabove, it is based upon the removal of evaporation heat, so that anadditional quantity of water must be added, which evaporates during thevacuum phase.

[0053]FIG. 4a) shows a conventional arrangement. A weighing container 25is shown which is suspended by means of a carrier structure 24 on aweighing cell 23. The weighing cell 23 measures the weight of theweighing container 25 including the carrier structure and content ofwater. When the valve 26 is opened, the water leaves the weighingcontainer 25 via an outlet pipe 27 and flows into an inlet pipe 30. Theinlet pipe 29 is connected rigidly to the pressurized container of themixer. The inlet pipe 30 and outlet pipe 27 are expediently surroundedby a pressure-resistant but flexible sleeve 29. To allow the water to beadded very quickly, water is drawn from the weighing container 25 andthe quantity is determined via the weight loss, which is detected by theweighing cell 23.

[0054] The pressure difference between the mixing chamber and thesurroundings or, in this case, the weighing container 25 can also beused advantageously in the context of the water supply, in order tosignificantly accelerate the charging process. This is possible in asimilar manner to the charging of mould sand described with reference toFIGS. 1 and 2, for example, if the mixing water is supplied while themixing container is under vacuum. However, with the arrangement shown inFIG. 4a), this is only possible subject to other disadvantages.

[0055] In the case of the arrangement shown in FIG. 4a, the vacuumpressure in the mixing container exerts a drawing force on the valve 26via the inlet pipe 30 with diameter D. This drawing force depends on themomentary pressure in the mixing container and has a disadvantageouseffect on the measuring accuracy of the weighing cell 23. Even thefilling of the weighing container 25 with water, during a process phase,in which no water is supplied to the mixing chamber, cannot be meteredaccurately, because the changing pressure in the mixing chamber is alsoalways exerted on the weighing the cell 23.

[0056] The particular version shown in FIG. 4b) provides that the valve26 is not arranged in the outlet pipe 26 but in and/or on the inlet pipe30. In this case, the sleeve 29 is necessarily above the valve 26 andnot, as in the case of conventional plants, below the valve 26.Moreover, this arrangement has the advantage that the falsifyinginfluence of the mixing chamber pressure on the weighing cell 23, on theone hand, occurs only while the valve is open, and on the other hand,the pressure acts on the weighing cell 23 only via the significantlysmaller cross-section d′ of the outlet pipe.

[0057] As a result of this arrangement, the weighing container 25 canreliably be filled with the desired quantity when the valve 26 isclosed. The weighing error while the valve is open can easily becorrected by means of a tare correction.

[0058] For particularly accurate dosages, the tare correction can becarried out with the assistance of the dosage computer 31 and the dosagemeter 33. The dosage meter 33 registers the current pressure in themixing chamber and passes this value to the dosage computer 31. Thedosage computer 31 calculates the drawing force exerted by the mixingchamber on the weighing cell 23 and corrects the weighing result, sothat the mixing water can be metered very accurately.

[0059] The duration of filling can be considerably reduced by exploitingthe pressure difference between the mixing chamber and the ambientpressure. For example, the cross-section d′ of the outlet pipe can bereduced, so that the falsifying influence of the drawing force can befurther reduced. As a result, the rate of filling is necessarilyincreased, but this is more than compensated by the vacuum-fillingprocess.

[0060] Introducing the mixing water under vacuum has the additionaladvantage that the water is immediately finely distributed and spreadsin a nebulous manner in the mixing chamber.

[0061] The thorough mixing of the mixing water with the mixture can befurther improved and above all accelerated if the mixing water issupplied via a device, as shown in FIGS. 5a) and 5 b). A mixing shaft 34with mixing tools 35 is provided in the mixer 1. The mixing shaft 34 ismounted outside the container in a bearing 32. Above the bearing, arotary connection 31 is connected to the inlet pipe 30. The waterflowing from the dosage device, preferably from the water weighingdevice described with reference to FIG. 4b), in the direction of thearrow, is guided via the rotary connection 31 into the longitudinal borehole 33 of the mixing shaft 34. The longitudinal borehole 33 isconnected at different heights by pipes or hoses 36 to outlet nozzles37. Subject to the vacuum prevailing in the mixing container, the wateris sucked through the supply and distribution system described directlyinto the mixture, without pumping or other conveying devices beingnecessary. The method according to the invention even allows theprocessing of recycled condensation water from a thermal exchange unitof the vacuum cooling process. Condensation water is generallycontaminated with fine particles, so that there can be no question ofcharging this water when using pumps or conventional nozzles, because apump will wear very rapidly as a result of the fine particles and thenozzles can frequently become clogged. According to the presentinvention, however, this water can be reused directly without priorcostly cleaning processes.

[0062] An alternative version of the present invention is shown in FIG.6. In this case, powdered additives are successfully utilized byexploiting the pressure difference (principle of suction conveyance)between the mixing container and the atmosphere.

[0063] These additives, often also referred to as quality-determiningmixture components, are normally blown into the mixer under pressure.However, for this purpose, appropriate pressure stores must be providedfor the pumping air. In addition to the undesirable additional spacerequirement, the consumption of expensive compressed air is not to beignored. Moreover, the vacuum cooling process cannot be implementedwhile the additives are being supplied, because the supply of additivesunder pressure is also necessarily associated with an increase inpressure in the mixing container. Furthermore, the charge of compressedair in the mixing chamber can have disadvantageous consequences. Inaddition to restricting the sealing function of the seal 8, as describedin the context of FIGS. 2 and 3, the charge of air can also delay theuniform thorough mixing of the mixture with the mixing water and theadditives.

[0064] According to the invention, the disadvantages described can beovercome by supplying the powdered additives with the assistance of apreferably stationary mixing tool 39 or its carrier arm 41. Thestationary mixing tool 39 is used primarily for guiding the material.With reference to the arrangement shown in FIG. 6, the mixing tool 39assumes the additional function of cleaning the container wall of mixer1. The mixer 1 or the mixing chamber rotates in an anticlockwisedirection as shown in FIG. 6 from above. The mixing tool “scrapes” alongthe container wall thereby cleaning any unmixed mixture components fromit. The mixing tool guides the mixture from the edge of the container tothe middle of the container 1. The mixing tool 39 is attached by meansof a carrying arm 41. The carrying arm 41 is designed to be hollow, sothat the powdered additives, the quantity of which has been determinedwith the assistance of the dosage weighing device 43, can be suppliedvia the feeder 42 into the hollow cavity 40 of the carrier arm. Theadditives are sucked into the mixing chamber as a result of the pressuredifference between the mixing container and the surroundings. The hollowcavity 40 is connected to a feeder nozzle 45, of which the outletopening is arranged in such a manner that the additives sucked in areguided inwards in as radial a manner as possible. The version shown inFIG. 6 exploits the suction action, which is formed in connection withthe mixing tool 39, in order to draw the additives inwards. For thispurpose, the mixing tool 39 provides yet another extended region 39′ inthe vicinity of the base, which is arranged in the direction of flow ofthe mixture essentially directly in front of the outlet opening of thefeeder nozzle 45.

[0065] As a result of this sophisticated arrangement and theexploitation according to the invention of the pressure difference,additives can be supplied simply and economically. Moreover, mixing isextremely effective and, above all, rapid.

[0066] The hollow tool designed for supplying additives can alsoadvantageously be used for ventilation, that is, for pressureequalization of the mixing container, when the vacuum cooling process iscompleted. For this purpose, air is simply sucked in through the feeder44 into the mixing container. Feeding the air directly into the mixture,that is, below the level of the mixture, provides the importantadvantage that the mixture is not temporarily compressed by theresulting pressure wave, which is not the case with the conventionalmixers, and the air can therefore be mixed into the mixture.

[0067]FIG. 7 shows an alternative version of the charging opening of themixer 1. In this version, the mixer 1 does not have a cover. Only apressure-resistant, rigid conveying funnel 46, which surrounds the inletopening, is provided. Above the conveying funnel, a similarlypressure-resistant but movable housing 47 is provided which is connectedvia a pressure-resistant, flexible connection 48 to the conveying funnel46. The weighing container 49 is used for the dosage of the mixture tobe added. The quantity to be filled can be deduced from the weight ofthe weighing container 49, which is determined via the force transducer51. At its lower end, the weighing container 49 provides apressure-tight closure cap 11, which can be opened and closed via anactivation lever 52. Additionally, closure mountings 51 are provided,which serve to hold the closure cap onto the weighing container 49 in avacuum-tight manner.

[0068] This version allows the addition of the mixture under vacuum. Thefilling procedure is as follows. Initially, the closure cap 11 of theweighing container 49 is closed. The mixing container 1 is evacuated sothat vacuum pressure prevails even inside the conveyor funnel 46 and thepressure-resistant housing 47. The mixture is then filled into theweighing container 49, and the quantity to be filled is determined viathe pressure transducer 50. When measuring the quantity to be filled, itshould be taken into account that the pressure difference between thehousing 47 and the interior of the weighing container 49 falsifies theweighing via the force transducer 50. This must be taken into accountwhen calculating the net weight. The weighing container 49 and thehousing 47 rigidly connected to the weighing container can readily bedisplaced in a vertical direction depending on the filling weight andthe pressure difference. This vertical movement is allowed by theflexible connection 48, which is clearly shown in FIG. 7 on the left inthe detailed enlargement.

[0069] In the next stage, the closure mountings 51, which enclose theclosure cap in the manner of a clamp, are rotated outwards around theaxis 53, as shown in FIG. 7 in the right-hand detailed view. The closurecap is thus unlocked and can then be opened with the assistance of theactivation lever 52. In combination with the large charging opening, thepressure difference between the solids weighing device and the mixingcontainer ensures rapid charging. Furthermore, a cover which includesthe associated, necessary drive units can be dispensed with in the caseof this version. Moreover, this version requires a lower structuralheight, because the rotational range for the mixing chamber cover is notnecessary, and the closure cap of the solids weighing device can bedesigned in such a manner that it is immersed in the conveyor funnel oreven in the mixing container opening during opening.

[0070] It is self-evident that all of the versions described can also berealized with smaller mixing container openings, although thisnecessarily means that the charging rate will be somewhat lower.Depending on the individual case of application, however, one of theversions described may be advantageous in combination with a smallercharging opening.

1. Method for the preparation of mould sand using a mixing process in amixer (1), the preparation taking place at least partially under vacuum,characterized in that the mould sand is supplied at least at times in avolume flow of at least 100 l/s through an opening in the mixer with across-sectional opening area of at least 0.25 m², preferably at least0.4 m², particularly preferably at least 0.5 m².
 2. Method according toclaim 1, characterized in that the pressure difference between theambient pressure and the pressure in one mixing chamber of the mixer (1)is used either as the sole or the predominant drive for at least onefeeding process for water or for one of the mixture components or toaccelerate the feeding process.
 3. Method according to claim 2,characterized in that at least one portion of the quality-determiningmixture components, the so-called additives, is introduced during themixing process.
 4. Method according to one of claims 1 to 3,characterized in that the individual mixture components are introducedone after another into the mixer (1) in a predetermined sequence. 5.Method according to one of claims 1 to 3, characterized in that water isonly introduced into the mixer (1) after the other mixture componentshave been introduced into the mixer (1) essentially concurrently. 6.Method according to any one of claims 1 to 5, characterized in that atleast one portion of the water is introduced directly into the mixturewith the assistance of a preferably rotating feeder device (34). 7.Method according to claim 6, characterized in that at least one portionof the water is introduced into the mixture via a feeder device, whichis connected to a mixing tool (34) or integrated in a mixing tool (34).8. Method according to one of claims 1 to 7, characterized in that thequality-determining mixture components are introduced into the mixer (1)below the filling level of the mixture.
 9. Method according to claim 8,characterized in that the quality-determining mixture components areintroduced essentially in a cylindrical, central region, of which theupper limit is essentially the filling level, of which the lower limitis the base of the mixer (1) and of which the radius represents amaximum of 90% of the radius of the mixing chamber.
 10. Method accordingto claim 8 or 9, characterized in that the quality-determining mixturecomponents are introduced into the mixer (1) in such a manner that theyprovide a motion component in a radial direction towards the centre ofthe mixer (1).
 11. Method according to any one of claims 1 to 10,characterized in that at least one portion of the quality-determiningmixture components is introduced into the mixer (1) after mixing withair.
 12. Method according to any one of claims 1 to 11, characterized inthat in order to ventilate the mixing chamber, the pressure is equalizedvia a feeder (45), which terminates in the mixing chamber below thefilling level of the mixture.
 13. Device for preparing mould sand with amixer (1), which provides a vacuum chamber (16) or is arranged in avacuum chamber (16), which can be closed in an essentially vacuum-tightmanner, with devices for feeding the components to be mixed, with atleast one mixing tool (34) and a device (38) for removing the readymixture, characterized in that at least one closable feeder connectionfor the mixture components exists or can be produced between the mixingcontainer and the outside, the feeder opening having a cross-sectionalopening area of at least 0.25 m², preferably at least 0.4 m², andparticularly preferably at least 0.5 m².
 14. Device according to claim13, characterized in that feeding takes place either exclusively throughthe pressure difference between the ambient pressure and the pressure inthe mixing chamber of the mixer or that feeding is at least acceleratedby this pressure difference.
 15. Device according to claim 13 or 14,characterized in that an essentially vacuum-tight, closable fillingopening of the mixer can be connected via a preferably vacuum-tightintermediate space (6, 6′) to the outlet opening of at least one feederdevice, which is preferably designed as a dosage weighing device (19).16. Device according to claim 15, characterized in that the fillingopening of the mixer and/or the output opening of the feeder deviceforms a cover (3, 11) with lateral cheeks (4, 11′) and which, in theopened state, forms a conveyor chute.
 17. Device according to claim 16,characterized in that a movable chute component (13, 13′) is provided,which is independent of a cover (3, 11).
 18. Device according to claim17, characterized in that a control device is provided, which,successively, for the purpose of feeding, firstly opens the cover (3) ofthe filling opening, then brings the movable chute component (13) into afunctional position, and then opens the cover (11) of the feedingdevice.
 19. Device according to any one of claims 13 to 18,characterized in that the mixing chamber (16) is arranged in apressurized container (17′) and that a closable air feeder (19) isprovided inside the pressurized container (17′), but outside the mixingchamber (16).
 20. Device according to claim 19, characterized in that acontrol unit is provided which opens the air feeder (19) when mixturecomponents are being supplied and closes the air feeder (19), when thevacuum container is closed in a vacuum-tight manner.
 21. Deviceaccording to any one of claims 13 to 20, characterized in that a feederdevice for water is arranged in such a manner that the water is fedthrough or along a preferably eccentrically arranged mixing tool (34)with mixing paddles (35) and is fed into the mixture essentially in theregion of the ends of the mixing paddles (37).
 22. Device according toany one of claims 13 to 21, characterized in that the fluid outletopening (37) in the feeder device for the water is arranged at differentdepths below the level of the mixture.
 23. Device according to any oneof claims 13 to 22, characterized in that the feeder device for thewater is a water dosage weighing device (25), the dosage weighing device(25) and the mixer being connected by an at least partially resilientpipe (27, 29, 30), which can be closed by a valve (26), the valve (26)being arranged directly on the mixer (1), so that the resilient portionof the pipe (29) is located between the valve (26) and the dosageweighing device (25).
 24. Device according to any one of claims 13 to23, characterized in that the supply of quality-determining mixturecomponents is provided below the level of the mixture with theassistance of a feeder lance (41).
 25. Device according to claim 24,characterized in that the outlet opening (45) of the feeder lance (41)is oriented tangentially to the direction of flow of the mixture andpreferably points in the direction of flow.